This invention relates to a method and apparatus for reducing ice forces on offshore platforms or other stationary marine structures installed in waters having moving ice sheets on their surface.
Substantial known reserves of oil and gas exist in Arctic regions, and many of these reserves lie below the surface of the Arctic Ocean or other bodies of water in the Arctic regions. In the past, techniques have been successfully used for tapping offshore oil and gas reserves in sub-Arctic regions by using either offshore platforms erected on the ocean floor, submersible moored drilling platforms, or moored drilling vessels. However, year-round use of these techniques in polar regions can be hazardous because of the ice problem which exists there during much of the year. For example, a permanent polar ice pack exists over much of the Arctic Ocean and varies in extent depending upon the time of year. During the Arctic winter the size of the ice pack expands to positions very close to and in some instances in direct contact with the shoreline. In those areas where the permanent ice pack does not come into direct contact with the shoreline, ice sheets which are fast to or fixed to the shoreline (known as land-fast ice) cover these areas and may extend 25 miles or more offshore.
The permanent ice pack slowly rotates and circulates in the Arctic Ocean, and land-fast ice also moves during the period in which it exists in the Arctic regions. The motion of a land-fast ice sheet is overall in random directions and in random amounts in response to tides, currents, winds and temperature changes. Ice sheets in the Arctic area may move as much as 60 feet per day and can exert substantial forces on a structure such as a drilling or production platform extending through the ice sheet from a supporting connection to the ocean floor. It is very difficult and expensive to construct an offshore structure which can withstand ice forces present in Arctic regions. Hence, there is a need to minimize the forces exerted on offshore structures so that year-round use of conventional offshore drilling equipment in Arctic regions will be possible. It is uneconomical to use conventional offshore drilling equipment in Arctic locations only during the short ice-free season because of the time and cost involved in moving the equipment into and out of position.
In the past, there have been disclosed a number of proposed solutions to the problem of reducing ice forces on offshore structures. For example, U.S. Pat. No. 3,831,385 discloses a stationary conical shroud surrounding the supporting structure of an offshore drilling platform from the sea floor to above the water surface. The conical shape of the shroud forces advancing ice sheets to ride up on the cone and break flexurally into a number of smaller pieces. The exterior surface of the cone is heated to produce a thin film of melted ice water between the ice and the heated conical surface so that the broken ice pieces will ride up on the cone rather than sticking to it. This system is suitable for relatively slow moving ice sheets, but for ice sheets moving at faster rates, the static cone structure may not break up the advancing ice fast enough to avoid transmitting substantial forces to the structure intended to be protected by the shroud from these forces. The static conical shroud also may not prevent ice broken from faster moving ice sheets from refreezing and accumulating in front of the shroud, which can obstruct the ramp-like surface and prevent the cone from continually breaking the advancing ice sheet. Also, the shroud described in U.S. Pat. No. 3,831,385 is practical only in relatively shallow water depths, and any given shroud structure is useful only in a narrow range of water depths; the shroud is secured to the sea floor to enable it, as compared to the structure it surrounds, to stand against the lateral loads imposed on it as an advancing ice sheet rides up along the conical surface at the waterline.
U.S. Pat. No. 3,779,019 discloses a number of submerged ice-breaking units which are spaced apart around and removed from a marine structure located in an advancing ice field. Each ice-breaking unit can be raised from its submerged location to exert an upward force from below the ice sheet to shatter the ice. However, once the ice is broken, it can refreeze, thereby requiring frequent operation of the several units to maintain reduced ice forces on the marine structure. For fast moving ice sheets, substantial amounts of broken ice pieces can accumulate on the upstream side of the marine structure.